What is the purpose of the Qur’an, and why is the most important thing/book to Muslims?

Excellent question -
The most simple answer is to guide humans.
Muslims believe God didn't just create us & leave, but that he actively guides us to be the best we can be.
This has come through many different signs & revealed books (like the Torah, & the Bible) when we were at different stages of readiness to accept guidance. The final revelation (the Quran) came to summarise, add to, & finalise the other books - God's final help & guidance to us.
Muslims believe it has the principle solutions to all base problems humans will face. This may include how to live a good personal life, a good family life, & develop as a thriving community (or country).
Humans (scholars - people who understand) need to take the principles & apply to the problems we see today. E.g. we may think technology such as the internet has changed all this, but it's just a tool. We still deal with the same problems like human emotions - just at a much faster rate because our communication tools (like the internet, Facebook etc.) are much faster.
It's a very deep topic that's too complex to fully explain here, but I hope that helped a little! ☺️